1. Field of the Invention
The present invention relates to an optical homodyne receiver, and more particularly a synchronous circuit for use in an optical homodyne receiver adapted to receive an optical signal modulated, for example, by BPSK (Binary Phase-Shift Keying).
2. Description of the Background Art
With a recent increase in optical communications capacity, active research has been concentrated on the phase modulation such as NRZ (Non-Return-to-Zero)-BPSK, which is more advantageous in signal-to-noise ratio than the conventional intensity modulation. Different from the conventional intensity modulation of directly modulating the intensity of a light, the phase modulation utilizes the coherence of light so as to render its phase convey information to be transmitted to thereby forward the information.
In coherent communication systems, receiver schemes such as heterodyne and homodyne detections have predominantly been proposed. With these proposed detection schemes, a receiver end prepares a carrier wave accurately synchronous in phase with a modulated signal received as input signal, and utilizes the interference therebetween to thereby demodulate the signal.
With the heterodyne detection, a beat signal caused by interference between a local oscillation light and a carrier wave slightly different in frequency from each other is detected to thereby determine the phase state of a received signal necessary for demodulation. The local oscillation light is generated by a local oscillation light source. With the homodyne detection, a receiver end generates a carrier wave accurately conformed in both frequency and phase to a received signal, and utilizes the interference therebetween to thereby determine the phase state of the received signal necessary for demodulation. These schemes can be implemented by means of the phase locking of a received input signal light to the local oscillation light. The heterodyne detection scheme does not require an accurate phase locking of the received input signal light with the local oscillation light, thus being considered to be higher in implementation. However, the heterodyne detection is lower in reception sensitivity than the homodyne detection by about 3 dB. These features are disclosed in Takanori Ohkoshi, et al., “Coherent Optical Fiber Communications”, published by Ohmsha, Ltd., Tokyo Japan, pp. 158-159, (1989).
In the past, many coherent communication systems based on the homodyne or heterodyne detection have been reported which have used an optical phase-locked loop applied to the frequency synchronization and phase locking of a received signal with a local oscillation light source provided on a receiver. One example of the reports is Stefano Camatel, et al., “10 Gbit/s 2-PSK Transmission and Homodyne Coherent Detection Using Commercial Optical Components”, ECOC2003, Vol. 3, We. P. 122, pp. 800-801.
When using such an optical phase-locked loop to demodulate, for example, a BPSK signal with a modulation index of 100%, the BPSK signal in itself does not include a spectral component of a carrier wave. It is therefore necessary to use some measure for extracting a phase difference of the carrier wave from the local oscillation light. In order to extract a phase difference of the carrier wave from the local oscillation light, in the wireless communications field, measures such as a multiply method and a Costas loop have been used for many years.
For example, in the BPSK system, a carrier wave is modulated in phase in response to binary values so as to be shifted by an angle of π (radian). When applying a multiply method of simply multiplying a carrier wave by, for example, twofold in frequency, the start phases 0 and π corresponding to the binary values of the carrier in a modulated signal are doubled to cause the phase differences 2π between the respective time slots of the binary values. Therefore, the periodicity of trigonometric functions causes the binary values in the frequency-doubled signal to have the same phase. As a result, a stable signal having a frequency equal to the doubled carrier frequency can be extracted by the multiply method.
With the Costas loop, a doubled phase difference can be extracted between a carrier wave and a local oscillation light. A circuit configuration where a Costas loop is applied to optical communications is disclosed in, for example, Y. Chiou, et al., “Effect of Optical Amplifier Noise on Laser Linewidth Requirements in Long Haul Optical Fiber Communication Systems with Costas PLL Receivers”, Journal of Lightwave Technology, Vol. 14, No. 10, pp. 2126-2134 (1996).
With an optical Costas loop receiver shown in FIG. 1 of Chiou, et al., in this solution, with respect to the position of 0 rad, signals sin (θ+d) and −cos (θ+d) are developed on the I-arm and Q-arm, respectively, of the circuitry, where θ represents a phase difference, and d does a stream of data, which takes its value of π/2 or −π/2 for every signal period. These signals are multiplied to thereby cancel a change in these signals. This derives sin 2θ=sin (2θ+2d) since this stream of data 2d has its value of π or −π. Signals can be obtained in this way, and thus used as a control signal for a phase-locked loop.
For radio communications, a carrier frequency uses a band enormously lower in frequency than in optical communications. Therefore, the above-described solutions are effective. However, in the optical communications using a carrier frequency of several hundred THz, it is difficult to use these solutions without modification.
In the multiply method, the BPSK signal having information conveyed on a carrier of several hundred THz needs to be multiplied “literally”. However, such multiplication is difficult to implement by means of currently existing electronic devices due to physical characteristic requirements of circuit components involved in the configuration. Instead, a nonlinear optical effect can be utilized to generate harmonics. However, the generation of harmonics involves many research issues remain unsolved such as difficulties about a wavelength region and conversion efficiency. Furthermore, for generating harmonics, when applying the BPSK system of having information directly conveyed on an absolute phase to modifying an optical signal by means of phase modulation effect such as nonlinear optical effect, phase information of a received signal light in itself may be caused to change. Therefore, this solution is difficult to reduce to practice.
In the case of Costas loop, a bottleneck in circuit configuration is a multiplier multiplying an I-axis and a Q-axis signal. This multiplier is required to accurately multiply output signals having components of an I-axis and a Q-axis having phases shifted by π/2. This requirement is caused by the fact that a result from the multiplying corresponds to a phase difference. Currently, available are multipliers which can accurately multiply signals in a low frequency bandwidth, and multipliers operable in a relatively high frequency bandwidth of several ten GHz.
In the multiplying process of such a multiplier, when the baseband of a signal to be demodulated does not entirely fall within a frequency band of the multiplier, a difference occurs between an actual or true phase error and a result from the multiplication. That results in a fluctuation in phase difference, deteriorating the stability of phase locking. Particularly, since the radio communications is far lower in bit rate than the backbone of an optical transmission system, multipliers capable of dealing with the entire baseband are easily available.
In the case of optical communications, however, the transmission rate available in backbone transmission lines is enhanced year by year so that the bit rates in the order of 40 Gbps become currently popular. Further, a telecommunications standard regulating signals of bit rates in the order of 100 Gbps is nowadays on the way of standardization. Such an increase in bit rate reveals waveform distortion and S/N (signal-to-noise) deterioration per length of transmission. In order to overcome the difficulties thus caused, as a modulation scheme in optical transmission the PSK (Phase-Shift Keying) system begins studied which is more advantageous in insensitivity against such waveform distortion and S/N deterioration than the conventional OOK (On-Off Keying) system. In order to apply the Costas loop stated above to such optical transmission systems, it would be difficult to implement phase locking unless the multiplier has its operable frequency band entirely falling within the baseband of a signal to be demodulated.
Furthermore, in the case of optical communications, due to using a high frequency signal of several ten Gbps, this multiplier is also required to have its input bandwidth of which the upper limit is correspondingly high to the baud rate of the high frequency signal dealt with. That is caused by the above-described cos (θ+d) and sin (θ+d) including a baseband signal component d. There may be a case where the baseband is so broad that the highest frequency included in the baseband component exceeds the upper limit of the multiplier operable. In that case, the I-axis and Q-axis signal components have the high frequency components, exceeding the operable band, cut off by a low-pass filter in the multiplier to thereby cause the signals per se to be smoothed. As a result, the multiplier develops on its output a result from multiplying these smoothed signals with each other, which does not accurately reflect the phase error. Therefore, with the optical communications, it is difficult to maintain a stable operation.
As understood from the above discussion, use of a Costas loop in an optical communications system having its capacity as large as several ten Gbps would require a multiplier which can multiply in high accuracy ultra-wide bandwidth signals ranging from a frequency almost close to a DC (Direct Current) component to be operable in the lowest frequencies included in the baseband components of a signal to be demodulated to a frequency of several ten GHz corresponding to the maximum frequencies included in the baseband components. However, currently available electronic device technology fails to easily configure such a multiplier. In researches on optical communications in the past, there are examples of researches relying upon a Costas loop for many years. However, seldom reports are found on an optical PLL (Phase-Locked Loop) which stably operates on a BPSK signal having a high bit rate over several ten bps. Therefore, some solution is expected which can configure a loop without placing a burden on an electronic device or using a multiplier.
Furthermore, in a Costas loop, when the I-axis and Q-axis components of a signal are not accurately in phase with each other, a result from the multiplying does not reflect the phase error. In order to prevent this, the circuit sections conveying these signal components need to be made accurately equal in length. In addition to this, the group delay characteristic of an amplifier or the like included in these sections also raises harmful effects. Therefore, this also needs to be prevented. Thus, such a solution of multiplying the outputs from the circuits symmetrically structured makes the circuit designing more difficult in practice, and has prevented dissemination of a homodyne system having the highest performance in optical coherent communication.